Method of making electric heating elements



April 1952 c. LACY-HULBERT ET AL 2,591,442

METHOD OF MAKING ELECTRIC HEATING ELEMENTS 2 SHEETS-SHEET 1 Filed Feb. 9, 1946 INVEHTORS W- V4.56 m

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P 1952 c. LACY-HULBERT ET AL 2,591,442

METHOD OF MAKING ELECTRIC HEATINGELEMENTS Filed Feb. 9, 1946 2 SHEET SSHEET 2 00aO OOOOOOOOObOOOObO OOOon hm mm O0 00 O 00 O plum; mm

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Patented Apr. 1, 1952 METHOD OF MAKING ELECTRIC HEATING ELEMENTS Cyril Lacy-Hulbert and Albert Victor Barton,

Oldbury, England, assignors to Simplex Electric Company, Limited, Oldbury, England, a

British company Application February 9, 1946, Serial No. 646,633 In Great Britain November 6, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires November 6, 1964 4 Claims.

This invention relates to the manufacture of electric heating elements of the kind comprising a length of resistance Wire or strip, usually of helically coiled form, embedded in compacted electrically insulating heat conducting material sheathed in a metal tube.

The usual method of constructing such heating elements is to place the resistance Wire or strip through the tube, plug one end of the tube with a metal plug, fill the tube from the other end with the electrically insulating but heat conducting material such as fused magnesia, plug the second end of the tube with a metal plug, and then pass the swaging tool along the whole length of the tube so as to compress the tube and so reducing the cross sectional area within the tube very considerably. After this swaging operation, the ends of the tube for the length of each of the plugs is cut off and connections are made to the ends of the resistance material so that current can be passed therethrough.

The present method of manufacture has certain disadvantages which are as follows:

I. The form and possibly the cross sectional area of the resistance material becomes modified by the pressure with the result that the resistance of the element as a whole is decreased. Further, the endwise movement between the element and the swaging tool causes some of the turns of the coil to be extended axially so that the turns of the coil towards one end of the element are less open than those at the other end. In effect when current is passed through the element, the glow of the element varies along its length, and it is not unknown to have some very bright portions and some dull portions.

II. The necessity for passing the whole length of the element through the swaging tools causes the metal plugs used at the ends to become compressed and elongated, and the pressure makes it impossible to use plugs of electrically insulating material, and the metal plugs used short circuit the resistance material to the outer tube. It is therefore necessary to cut off a length at each end of the tube to get rid of plugs. This involves wasting a length of the resistance material at each end, and it involves the necessity of making two fresh electrical connections to the resistance material, one at each end of the element.

III. The length of the element is substantially increased during the swaging operation.

IV. The swaging operation which is done in a plurality of stages causes work hardening of the tube so that an annealing operation is necessary between each two swaging operations and be- 2 tween the last swaging operation and the bending of the tube into its final form.

The object of the present invention is to avoid one or more of these disadvantages.

According to the present invention the compacting of the insulating powder is achieved by placing the tube containing the element and the insulating powder between form tools moving transversely to the length of the tube and operating to reduce the outside diameter of the tube thereby causing the metal so displaced to form one or more ribs which extend along the tube.

For compressing a substantial length of element, the form tools or dies are caused to close and open periodically, and the element is automatically fed parallel to its length with a step by step movement, such feeding movements taking place only when the dies are open.

The length of the feeding step is preferably rather less than the length of the dies, so that there are no portions of th element which remain uncompressed except just at the ends where the plugs are situated.

It is a further feature of the present invention to plug the ends of the tube with plugs of electrically insulating material, the relatively heavy section terminal wire attached to the resistance wire extending through these plugs, and the compressing operation being limited to that part of the element which is disposed between the inner end of one plug and the inner end of the other plug, so that the pressure is not applied to the end portions of the element where the plugs are situated. With this arrangement the wastage of a length of relatively heavy section terminal wire, end plugs and tube at each end of the element is avoided, and the necessity for making electrical connections at each end of the element is also avoided.

Referring to the drawings:

Figure 1 is a sectional view showing a, cross section of the element before it has been compressed and showing the dies in an open position.

Figure 2 is a view similar to Figure 1, but showing the element compressed and the dies closed.

Figure 3 is a view in side elevation showing one end'of the element.

Figure 4 is a sectional view showing one end of the element.

Figure 5 is a sectional view in side elevation showing one form of the apparatus that may be used for carrying the method into practice.

The resistance element I0 may be formed as a coil, and it is placed in a metal tube II which may be made of the alloy known as Inconel or any other suitable metal. The space within the element Ill and between the coils of the element I and between the exterior of the element l0 and the wall of the tube H is packed with a suitable electrically insulating but heat conducting material, such for instance as fused magnesia. The material is shown at I2.

Secured to each end of the resistance coil is a terminal pin l3. These terminal pins project through the ends of the element and are adapted to have secured to them stranded conducting members M.

The ends of the tube are then plugged temporarily. The plugs may be made of vulcanized rubber and are inserted in the ends 'of the tube, the plugs .being provided with central holes through which the terminal pins project. These plugs are for the purpose of preventing endwise movement of the insulating powder. They do not however prevent the escape of air which passes out during the compacting operation.

Instead of plugging both ends of the tube temporarily in this manner one end of the tube may be plugged and sealed permanently as shown in Figure 4 and the opposite end plugged temporarily in a manner to allow air to escape. It is however preferred to plug both ends temporarily.

After both ends of the tube have been plugged in this way, the tube is placed in a machine such as that shown in Figure 5, this machine incorporating a lower die I9 upon which a part of the tube rests, and an upper die fixed to the reciprocating ram 2] of a press.

Stripper plates 22, 23 are mounted on the machine, and after each pressing operation, these stripper plates are given a short upward movement to free the pressed part of the tube from the lower die. This movement may be imparted to the stripper plates by any suitable means operated by a cam or other means from the ram or shaft of the press or on any other available shaft driven by the machine.

For instance such operating means may comprise a cam on the shaft of the press or on a shaft driven intermittently by the ram or a member moving therewith.

The ram of the press is further provided with a rack 24 gearing with a pinion 25 upon a shaft 26, this shaft 26 carrying a sprocket wheel 21 coupled by a chain 28 to a free wheel sprocket 29 on a shaft 30. The shaft 30 carries a second sprocket 3! on which is mounted a chain 32 engaging a sprocket 33 on a shaft 34 carried by 'a suitable support 35. The chain 32 is used for operating a gripper 36 in which the leading end of the tube II is detachably secured. The gripper 36 is provided with a pin 31 engaging the chain, and this pin is mounted for sliding movement relatively to the gripper. The pin is acted upon by a compression spring 38, and the arrangement is such that should the chain 32 commence to move before the tube has been freed from the lower die, the pin 3'! can move a short distance by compressing the spring 33 without moving the gripper.

On upward movement of the ram of the press, the sprocket chain 28 is driven and drives the shaft 30 so that the chain 32 is driven to cause the tube I I to move to the left.

The stripper plates 22 and 23 are operated by suitable means to lift the tube l I out of the lower die at the commencement of the upward movement of the upper die, but having lifted the tube clear of the lower die, the stripper plates cease to move upwardly while the tube is travelling to the left. At the commencement of the downward movement of the upper die, the stripper plates return to their lower position and the length of the tube about to be pressed is lowered on to the lower die. Preferably the stripper plates move upwardly just far enough to lift the tube clear of the lower die and then they stop while the upper die moves upwardly and the tube is fed in an endwise direction. In this way the stripper plates serve to free the tube both from the lower die and the upper die.

During the downward movement of the ram of the press, the rack 24 drives the pinion 25 so that the chain 28 is driven, but no motion is imparted to the shaft 30 as the sprocket 29 has a free wheel connection thereto.

The gripper 36 is supported on a rail 39, and one end of the gripper is provided with an incline 40 adapted to engage a fixed incline 4| on the rail, so that the pin 3'! is lifted out of the chain when the required length of tube has been treated.

The tube l l is supported on a roller 42 mounted on a shaft 43 supported in lugs 44 on the bed 45 of the machine, and a further supporting rail for the right hand end of the tube may be provided to the right of the machine, this rail being provided with suitable supports.

The pressing operation is not applied to the two ends of the tube where the temporary plugs are provided, but is confined to that portion of the tube between these plugs. The pressing operation is carried out in a succession of strokes, a suitable length of the tube adjacent the leading end being pressed first, and the upper die then rising and the tube being automatically fed forward by a distance somewhat shorter than the length of the tube which has been pressed. Then a second pressing operation is effected, and so on, until the tail end of the tube is reached.

After the pressing operation is complete the temporary plugs are removed from the ends of the tube and the ends of the tube are then permanently sealed by inserting sealing and insulating washers [5 which may be made of glass and which may be fused to the interior of the tube. Additional insulating plugs l6 are then inserted in the ends of the tube. The plugs may be made of steatite and at a suitable position on their peripheries they are provided with indentations I1 into which the metal of the tube is forced locally as shown at 18.

As will be: seen from Figures 2 and 3, the pressing operation results in the formation of two longitudinal ribs 46, one at each side of the tube.

The method of manufacture in accordance with this invention avoids any appreciable elongation of the tube during the process and has the further advantage that as the dies operate with a relatively slow squeezing action little or no work hardening of the metal of the tube is produced so that annealing operations are unnecessary.

What we claim then is:

1. A method of making electric heating elements of the kind specified wherein a metal tube of circular section containing an axially disposed coil resistance element and filled with compacted electrically insulated heat conducting material in powder form is first temporarily sealed at each end by a plug which prevents endwise movement of the insulating powder but which allows the escape of air, and the tube is then subjected to pressure from two form tools acting on opposite sides of the tube, one of which moves towards the other in a direction at right angles to the axis of the tube, each form tool having a straight groove of part circular section and of smaller radius than that of the cross section of the tube, and a rebate at each side of the groove of a depth equal to the thickness of the metal of the tube, the tube being thus contracted in diameter so as to compact the insulating material further, the surplus metal of the tube on each side thereof being squeezed by the form tools into a longitudinal rib of U section with the two sides of the rib in contact, said rib being formed in the rebate, the pressing operations being confined to that part of the tube between the inner ends of the plugs at the ends of the tube, so that pressure is not applied to the plugged parts of the tube, said plug being removed after the pressing operation and the end of the tube permanently sealed by inserting in the tube a sealing in insulating washer which is fused to the interior of the tube.

2. A method of making electric heating elements of the kind specified wherein a metal tubeof circular section containing an axially disposed coil resistance element and filled with compacted electrically insulating heat conducting material in powder form is first temporarily sealed at each end by a plug which prevents endwise movement of the insulating powder but which allows the escape of air, and the tube is then subjected to pressure from two form tools acting on opposite sides of the tube, one of which moves towards the other in a direction at right angles to the axis of the tube, each form tool having a straight groove of part circular section and of smaller radius than that of the cross section of the tube, and a rebate at each side of the groove of a depth equal to the thickness of the metal of the tube, the tube being thus contracted in diameter so as to compact the insulating material further without materially changing the cross sectional shape of the mass of tube on each side thereof being squeezed by the form tools into a longitudinal rib of U section with the two sides of the rib in contact, the pressing operations being confined to that part of the tube situated between the inner ends of the plugs at the ends of the tube, so that pressure is not applied to the plugged parts of the tube, said plug being removed after the pressing operation and the end of the tube permanently sealed by inserting in the tube a sealing and insulating washer which is fused to the interior of the tube.

3. A method of making electric heating elements of the kind specified wherein a metal tube of circular section containing an axially disposed resistance element formed as a helical coil and filled with compacted electrically insulating heat conducting material in powder form is first temporarily sealed at each end by a plug which prevents endwise movement of the insulating powder but which allows escape of air, and the tube is then subjected to pressure from two form tools acting upon opposite sides of the tube, one of which moves towards the other in a direction at right angles to the axis of the tube, each form tool having a straight groove of part circular section and of smaller radius than that of the cross section of the tube, and a rebate at each side of the groove of a depth equal to the thickness of the metal of the tube, the tube being thus contracted in diameter so as to compact the insulating material further without materially changing the cross sectional shape of the mass of insulating material or that of the resistance element, the surplus metal of the tube on each side thereof being squeezed by the form tools into a longitudinal rib of U section with the two sides of the rib in contact, the pressing operation being confined to that part of the tube situated between the inner ends of the plugs at the ends of the tube, so that pressure is not applied to the plugged parts of the tube, said plug being removed after the pressing operation and the end of the tube permanently sealed by inserting in the tube a sealing and insulating washer which is fused to the interior of the tube.

4. A method of making electric heating elements of the kind specified wherein a metal tube of circular section containing an axially disposed resistance element formed as a helical coil and filled with compacted electrically insulating heat conducting material in powder form is first temporarily sealed at each end by a plug which prevents endwise movement of the insulating powder but which allows escape of air, and the tube is then subjected to pressure from two form tools acting upon opposite sides of the tube, one of which moves towards the other in a direction at right angles to the axis of the tube, each form tool having a straight groove of part circular section and of smaller radius than that of the cross section of the tube, and a rebate at each side of the groove of a depth equal to the thickness of the metal of the tube, the tube being thus contracted in diameter so as to compact the insulating material further without materially changing the circular form of the end aspect of the resistance element, the surplus metal of the tube on each side thereof being squeezed by the form tools into a longitudinal rib of U section with the two sides of the rib in contact and the annular space between the exterior of the resistance element and the inner wall of the tube being filled with highly insulating material the Surplus metal of the compacted insulating-material of constant thickness, the pressing operation being confinedto that part of the tube between the inner ends of the plugs at the ends of the tube, so that pressure is not applied to the plugged parts of the tube, said plug being removed after the pressing operation and the end of the tube permanently sealed by inserting in the tube a sealing and insulating washer which is fused to the interior of the tube.

CYRIL LACY-HULBERT. ALBERT VICTOR BARTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 810,241 Wickstrom Jan. 16, 1906 1,727,895 Mraz Sept. 10, 1929 1,903,604 Wiegand Apr. 11, 1933 2,038,535 Brenizer Apr. 28, 1936 2,157,884 Backer May 9, 1939 2,178,141 Frame Oct. 31, 1939 2,205,893 Vuger June 25, 1940 2,280,352 Penfold et a1 Apr. 21, 1942 2,284,862 Ginder June 2, 1942 2,371,469 Rogoff Mar. 13, 1945 2,375,058 Wiegand May 1, 1945 2,462,016 Wiegand Feb. 15, 1949- FOREIGN PATENTS Number Country Date 418,155 Great Britain Oct. 19, 1934 592,124 Great Britain Sept. 9, 1947 

